1. Technical Field of the Invention
This invention relates generally to integrated circuits and more particularly to reducing substrate noise coupling.
2. Description of Related Art
FIG. 1 is a cross-sectional view of a prior art N-channel transistor fabricated on a substrate. As shown, the N-channel transistor includes two N-doped implants to produce a source and drain. The N-channel transistor further includes a gate and a P-doped implant that functions as a guard ring. To activate the N-channel transistor, a voltage VDS is provided to the drain and a voltage VGS is provided to the gate.
In many applications, the N-channel transistor may be used in noise sensitive circuitry such as amplifiers, buffers, analog-to-digital converters, et cetera. As is often the case, an integrated circuit has millions of transistors on its substrate, some of which are used in digital circuitry that produces noise, which is coupled into the substrate.
In the illustration of FIG. 1, a portion of a transistor (e.g., a gate and drain) that may be used in digital circuitry is shown on the substrate. The transistor within the digital circuitry is switched on and off using, for example, 1-volt VGS voltage, which produces a varying voltage between the supply voltage and AC or DC ground at the transistor's drain. Such a varying voltage produces an AC voltage gradient with respect to the guard ring of the transistor used in the noise sensitive circuit. The gradient is represented by the thin dashed lines.
The substrate may include different regions as shown. For example, one region may be a P-doped region that has a relatively low resistivity (for example, 0.1 OHMS-centimeter) and a lightly doped P-region which has a higher resistivity (e.g., 20 OHMS-centimeter). Due to the voltage gradient and the impedance of the P-doped region, AC noise voltage couples from various terminals (e.g., drain and/or gate) of the transistor in the noise generating circuit to various terminals (e.g., drain and/or gate) of transistor of the noise sensitive circuit.
In particular, the substrate coupled noise causes the voltage of the drain and/or gate of the transistor of the noise sensitive circuit to vary, which alters its operating point. As such, the substrate coupled noise modulates the signals being processed by the transistor as is desired function within the noise sensitive circuit causing adverse affects on the overall performance of the noise sensitive circuit.
Therefore, a need exists for isolating substrate coupled noise within integrated circuits.